Stabilized tunnel junction component

ABSTRACT

A method for stabilizing a tunnel junction component, in which a mask is formed on the surface of a substrate, and conductors are constructed by evaporation onto the substrate in an evaporation chamber, and at least one thin oxide layer element is oxidized on top of a selected conductor. This remains partly under the following conductor, thus forming a tunnel junction element with those conductors and titanium (Ti) or another gettering substance is evaporated on top of the said following conductor, before the tunnel junction component is removed from the evaporation chamber, when the titanium layer thus created protects the tunnel junction element from the detrimental effects of air molecules.

TECHNICAL FIELD

The present invention relates to a method for stabilizing a tunneljunction component and a corresponding stabilized tunnel junctioncomponent, in which method a mask is formed on the surface of asubstrate and conductors are built on the substrate by evaporation andat least one thin oxide layer element is oxidized on top of a selectedconductor and remains partly under the next conductor, thus forming atunnel junction element between these conductors, and which element isprovided with contacts and cased.

BACKROUND OF THE INVENTION

U.S. Pat. Nos. 5,947,601 and 5,974,806 disclose certain thin filmconstructions exploiting tunnel junctions. It has been observed thatambient air often has a corrosive effect on nano andless-than-micron-sized thin film conductors. The long-term instabilityis largely due to the large surface-area/volume ratio of these smallstructures. This problem acquires central importance with the increasein the number of nano-electronic sensors, which should remain stable fora number of years. This instability has been quite clearly observed inthe ‘Coulomb blockade’ nano thermometer (CBT) and micro cooler disclosedin the publications referred to above. It has become apparent thatnormal hermetically sealed cases for microelectronic sensors and othercomponents are unsuitable and insufficient in these cases, so that theaging problem requires a more developed solution.

The variation in the resistance of a CBT sensor signifies an increase indispersion in the junction parameters, thus reducing the absoluteaccuracy. A relative change of 10% in the resistance of the sensor hasbeen observed to cause a maximum change of 0.5% in absolute accuracy.

Sensors and other components of this kind cannot generally be installedin metal capsules, as this increases the size and mass, as well asdemanding non-magnetic materials and involving a risk of overheatingwhen closing the capsule. It is also difficult to use protective gases,because an extremely high degree of purity would be required, which isdifficult to maintain in a small volume. It is also difficult to createand maintain a sufficient vacuum in a small volume.

It is difficult to use a coating substance to protect a component, asthe protection cannot be carried out before removing the evaporationmask, which requires the component to be taken from the evaporationchamber to an air space, which would mean the continuation of agingafter manufacture, despite the coating protection.

In the above publications, a line width of typically 200 nm, generally0.01-10 micrometers (10-10 000 nm), is used in the tunnel junction. Inthe thermometer, the length of the tunnel junction is typically 300 nm.The thickness of the oxide forming a tunnel junction is extremely small,typically only about 1 nm. This is considerably less than the sizes, forexample, in the P and N elements doped in a silicon chip, which are usedin conventional microcircuits (memories, processors, and digital andanalog circuits in general).

SUMMARY OF THE INVENTION

The present invention provides a manufacturing method for stabilizingtunnel junction components and a stabilized tunnel junction component,which will solve the aging problem. The characteristic features of amanufacturing method according to the invention in which a mask isformed on the surface of a substrate, and conductors are constructed byevaporation onto the substrate in an evaporation chamber, and at leastone thin oxide layer element is oxidized on top of a selected conductor,which remains partly under the following conductor, thus forming atunnel junction element with those conductors, and which tunnel junctioncomponent is provided with contacts and cased, is characterized in thattitanium (Ti) or another gettering substance is vaporized on top of thesaid following conductor, before the tunnel junction component isremoved from the evaporation chamber, when the titanium layer thuscreated protects the tunnel junction element from the detrimentaleffects of air molecules, both in the later stages of the manufacture ofthe tunnel junction component and as a finished component.

Correspondingly, the characteristic features of a tunnel junctionaccording to the invention in which there are conductors on the surfaceof the substrate and at least one oxide layer between two overlappingconductors, thus forming a tunnel junction element with them, and inwhich the tunnel junction component includes contacts to connect it toexternal circuits, is characterized in that there is a titanium layer(Ti) on top of the uppermost conductor.

According to the invention, the tunnel junction can be protectedimmediately after manufacture, before the tunnel junction component isremoved from the evaporation chamber. The titanium layer appears to havethe effect that the thin oxide layer is no longer able to grow, onaccount of the air molecules, especially the oxygen and water molecules,because the titanium layer gathers the molecules into itself. Some othersubstance, particularly a metal, with similar gettering properties canbe used. The reactivity of the getter must be essentially greater thanthat of the substance used in the conductor layer. In the same way astitanium (Ti), the other substance absorbs the oxygen and watermolecules in the air into itself. However, it is important that thematerials used in manufacture and the uncased component are not exposedto moisture.

According to one preferred application, the uppermost conductor isoxidized before the titanium layer is evaporated, so that the titaniumlayer cannot substantially affect the electrical values of the circuit.According to a second preferred application, the titanium layer iscoated already in the same stage with a layer of copper or other metal,so that the titanium layer does not become saturated when the tunneljunction component must be removed from the evaporation chamber toremove the mask.

In the following, the invention is described with reference to theaccompanying figures, which show the problem underlying the inventionand the construction of a tunnel junction component according to theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the problem situation in a tunnel junction componentaccording to the prior art.

FIG. 2 shows the construction of a tunnel junction of the tunneljunction component according to the invention in detail.

DETAILED DESCRIPTION OF THE INVENTION

The tunnel junction 2 according to FIG. 1 is quite small in size. Theoverlapping part of conductors 3 and 4 formed on substrate 1 istypically only 200×300 mm. The thickness of the oxide layer formed ontop of the bottommost conductor is even less, being only about 1 nm.This thin layer of oxide then remains to insulate conductors 3 and 4from each other. The stability of tunnel junction 2 depends greatly onthe stability of this oxide layer. According to FIG. 1, it is apparentthat oxygen (O₂) and water (H₂O) molecules penetrate tunnel junction 2from the side and thicken the oxide layer, causing its properties todeteriorate over time. As such, the precise mechanism of this effect isnot known.

In this case, the substrate of the tunnel junction component is passiveand does not affect the electrical properties of the component, FIG. 2.Silicon (Si) is generally used as the substrate 1, on the surface ofwhich a thin (0.1-2 μm) layer 11 of silicon nitride (Si₃N₄) or siliconoxide (SiO₂) is grown. However, substrate 1 can be, for example, quartzor some other passive material.

The first conductor 3 is vaporized to the surface of the substrate bymeans of two angle evaporation. A thin oxide film 5 is formed on top ofthis by permitting a small amount of oxygen to enter the chamber. Afterthat, the evaporation angle is changed and next conductor layer 4 isevaporated, partly overlapping the first conductor layer 3 in a mannerthat is, as such, known. Tunnel junction 2 is then formed between, andin conjunction with the overlapping conductor components 3 and 4. Afterthe evaporation of the second conductor layer 4, a small amount ofoxygen is again allowed to enter the evaporation chamber, when a secondoxide layer 6 is formed. The second oxide layer 6 is used to preventconductor component 4 and titanium layer 7 diffusing each other. Next,titanium 7 is evaporated at the same angle as the second conductor 4 wasevaporated. Finally, copper 8 is evaporated at the same angle to protectthe titanium 7, so that it will not saturate at room temperature. Afterthis, the tunnel junction component can be removed from the evaporationchamber and its mask dissolved off. Finally, the tunnel junctioncomponent is put back in the evaporation chamber and a uniform layer ofsilicon monoxide 9 (SiO) is evaporated on top of the component. Afterthis, the tunnel junction component can be removed from the chamber,provided with contacts, and cased in a manner that is known.

Generally, aluminium is used as the conductors 3 and 4, becausealuminium reacts with oxygen to form a sufficiently thin and uniformoxide layer on its surface. As such, some other conductor may be used,for example, niobium (Nb).

The titanium layer 7 is 5-1000-nm thick, preferably 20-50 nm. A copperlayer 8, with a thickness of 5-100 nm, preferably 20-50 nm, is used ontop of titanium layer 7. The dimensions of the length and width oftunnel junction 2 are preferably in the range 50-2000 nm. The depth ofthe oxide layer 5 of tunnel junction 2 is preferably in the range 0.1-5nm, most preferably 0.5-2 nm.

The oxide layer is usually aluminium oxide Al₂O₃, but other oxidecompounds may be considered. The width of the line, which above wastypically 0.2 μm, can be within the range 0.01-2 μm.

It is unclear how broadly the titanium layer 7 spreads to the sides ofthe junction and what is the exact mechanism of the effect, but in teststunnel junction components according to the invention have retainedtheir electrical properties unaltered over a test period of several tensof days, whereas the electrical properties of previous sensors began toalter immediately after manufacture, which made their use difficult.

As such, the method according to the invention can be adapted within thescope of the claims in many ways. For example, it is naturally possibleto use multi angle evaporation instead of the above two angleevaporation.

Although the invention has been described by reference to a specificembodiment, it should be understood that numerous changes may be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiment, but that it have the full scope defined by thelanguage of the following claims.

What is claimed is:
 1. A stabilized tunnel junction component, in whichthere are conductors on the surface of the substrate and at least oneoxide layer between two overlapping conductors, thus forming a tunneljunction element with them, and in which the tunnel junction componentincludes contacts to connect it to external circuits, characterized inthat there is a titanium layer (Ti) on top of the uppermost conductor.2. A tunnel junction component according to claim 1, characterized inthat there is a thin oxide layer between the titanium layer and theuppermost conductor.
 3. A tunnel junction component according to claim1, characterized in that the thickness of the titanium layer is 5-100nm.
 4. A tunnel junction component according to claim 1, characterizedin that there is a copper layer, of a thickness of 5-100 nm, on top ofthe titanium layer.
 5. A tunnel junction component according to claim 1,characterized in that in the tunnel junction component there is asilicon substrate, on the surface of which there is an insulating layer.6. A tunnel junction component according to claim 5, wherein theinsulating layer is a layer of silicon nitride Si₃N₄.
 7. A tunneljunction component according to claim 1, characterized in that the widthand length of the tunnel junction are within the range 50 nm -300 mu. 8.A tunnel junction component according to claim 1, characterized in thatthe depth of the oxide layer of the tunnel junction is in the range of0.1 nm-5 mu.